全文获取类型
收费全文 | 2739篇 |
免费 | 219篇 |
国内免费 | 334篇 |
专业分类
化学 | 516篇 |
晶体学 | 5篇 |
力学 | 72篇 |
综合类 | 114篇 |
数学 | 2159篇 |
物理学 | 426篇 |
出版年
2024年 | 4篇 |
2023年 | 15篇 |
2022年 | 53篇 |
2021年 | 59篇 |
2020年 | 66篇 |
2019年 | 56篇 |
2018年 | 67篇 |
2017年 | 68篇 |
2016年 | 75篇 |
2015年 | 52篇 |
2014年 | 121篇 |
2013年 | 242篇 |
2012年 | 120篇 |
2011年 | 178篇 |
2010年 | 144篇 |
2009年 | 191篇 |
2008年 | 195篇 |
2007年 | 195篇 |
2006年 | 162篇 |
2005年 | 145篇 |
2004年 | 154篇 |
2003年 | 143篇 |
2002年 | 136篇 |
2001年 | 97篇 |
2000年 | 73篇 |
1999年 | 94篇 |
1998年 | 80篇 |
1997年 | 72篇 |
1996年 | 41篇 |
1995年 | 32篇 |
1994年 | 30篇 |
1993年 | 15篇 |
1992年 | 17篇 |
1991年 | 11篇 |
1990年 | 17篇 |
1989年 | 13篇 |
1988年 | 11篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1974年 | 2篇 |
1970年 | 1篇 |
1959年 | 1篇 |
1936年 | 2篇 |
排序方式: 共有3292条查询结果,搜索用时 15 毫秒
941.
On Minimal Realization Over a Finite Chain Ring 总被引:1,自引:0,他引:1
Graham Norton 《Designs, Codes and Cryptography》1999,16(2):161-178
Let R be a finite chain ring, e.g. a Galois ring. We give a compact recursive formula for a minimal realization of a finite R-sequence. In particular, we show how to obtain a monic minimal polynomial and a rational approximation of a finite R-sequence. We also show how to solve the classical key equation of Algebraic Coding Theory over R. 相似文献
942.
Raffaele Vitolo 《Differential Geometry and its Applications》1999,10(3):1191-255
We consider two geometric formulations of Lagrangian formalism on fibred manifolds: Krupka's theory of finite order variational sequences, and Vinogradov's infinite order variational sequence associated with the
-spectral sequence. On one hand, we show that the direct limit of Krupka's variational bicomplex is a new infinite order variational bicomplex which yields a new infinite order variational sequence. On the other hand, by means of Vinogradov's
-spectral sequence, we provide a new finite order variational sequence whose direct limit turns out to be the Vinogradov's infinite order variational sequence. Finally, we provide an equivalence of the two finite order and infinite order variational sequences modulo the space of Euler-Lagrange morphisms. 相似文献
943.
944.
张石生 《应用数学和力学(英文版)》1999,20(12):1310-1318
The purpose of this paper is to study the Mann and Ishikawa iterative approximation of solutions for m-accretive operator equations in Banach spaces. The results presented in this paper extend and improve some authors’ recent results. 相似文献
945.
The direct production of copper sulphate from copper ore is an important route to recover copper. The condition, however, is dependant on temperature and sulphatising environment. The oxidation of chalcopyrite in static air bed condition has been studied by TG/DTA and DSC techniques. The addition of catalysts, improved the sulphation by in-situ producing better condítions. The mutual effects of sulphides were further confirmed by studying the oxidation reaction on pure copper-iron sulphides and results so obtained were corroborated with X-ray diffractrograms. With only chalcopyrite a mass gain of 8% (TG) corresponding to copper sulphate formation was observed, in the temperature range 628–738 K. The TG plots showed respective mass gain of 14, 17 and 12% in presence of Fe2O3, Na2SO4 and FeSO4 with chalcopyrite in the wider temperature range 628–923 K. As such the cupric sulphide had a negligible tendency of sulphation, which increased with the addition of ferrous sulphide mixture under the temperature range studied. At higher temperature copper ferrite formation was found.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
946.
D. Sy C. Durand S. Hugot C. Savoye C. Swenberg M. Charlier M. Spotheim-Maurizot 《Theoretical chemistry accounts》1999,101(1-3):114-120
Sequence-dependent variations of DNA structure modulate radiation-induced strand breakage. Thiols reduce breakage by scavenging
damaging radiolytic OH
.
and repairing sugar radicals. As shown by sequencing gel electrophoresis, WR-1065 radioprotection is modulated by sequence,
whereas that of WR-151326, a larger thiol, is more evenly distributed. Molecular modelling was performed on complexes of a
53 bp oligonucleotide (belonging to a natural restriction fragment) with one molecule of WR-1065 or WR-151326. Energy minimised
structures exhibit a broadening of the minor groove of an AAATT motif upon WR-1065 binding, and a narrowing of the groove
upon WR-151326 binding. Consequently, the accessibility to OH˙ of H4′ (whose abstraction leads to strand breakage) increases near WR-1065, whereas it decreases near WR-151326. This modifies
locally the otherwise homogeneous radioprotection. The effect of WR-151326 strengthens the protection at all tested binding
sites, whereas that of WR-1065 diminishes it in some regions, in good agreement with the observed radioprotection distribution.
Received: 24 April 1998 / Accepted: 4 August 1998 / Published online: 11 November 1998 相似文献
947.
Eric Hughes Eric B. Brouwer Robin K. Harris 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1999,138(2):256
For compounds giving “crowded” 1-dimensional magic-angle-spinning spectra, information about the local atomic environment in the form of the chemical shift anisotropy (CSA) is sacrificed for high resolution of the less informative isotropic chemical shift. Magic-angle-turning (MAT) NMR pulse sequences preserve the CSA information by correlating it to the isotropic chemical shift in a 2-dimensional experiment. For low natural abundance nuclei such as 13C and 15N and under 1H heteronuclear dipolar decoupling conditions, the dominant NMR interaction is the chemical shift. For abundant nuclei such as 1H, 19F, and 31P, the homonuclear dipolar interaction becomes a significant contribution to the observed linewidth in both F1 and F2 dimensions. We incorporate MREV8 homonuclear multiple-pulse decoupling sequences into the MAT experiment to give a multiple-pulse MAT (MP-MAT) experiment in which the homonuclear dipolar interaction is suppressed while maintaining the chemical shift information. Extensive use of computer simulation using GAMMA has guided the pulse sequence development. In particular, we show how the MREV8 pulses can be incorporated into a quadrature-detected sequence such as MAT. The MP-MAT technique is demonstrated for a model two-site system containing a mixture of silver trifluoroacetate and calcium difluoride. The resolution in the isotropic evolution dimension is improved by faster sample spinning, shorter MREV8 cycle times in the evolution dimension, and modifications of the MAT component of the pulse sequence. 相似文献
948.
Ph. Barbeau J. F. Gerard B. Magny J. P. Pascault 《Journal of Polymer Science.Polymer Physics》2000,38(21):2750-2768
In this study, we investigated the role of diisocyanate on the properties of polyurethane acrylate (PUA) prepolymers based on polypropylene oxide (M̄n = 2000 g · mol−1). The diisocyanates studied were isophorone diisocyanate, 4‐4′dicyclohexylmethane diisocyanate, and toluene diisocyanate (pure 2,4‐TDI, pure 2,6‐TDI, and a TDI mixture, TDItech). The molecular structure of the diisocyanate had a major role on the course of the polycondensation and, more precisely, on the sequence length distribution of the final prepolymer. Moreover, the structural organization of the prepolymer also strongly depended on the nature of the diisocyanate. Two types of behaviors were particularly emphasized. On the one hand, the PUA synthesized from 2,4‐TDI displayed an enhanced intermixing between soft polyether segments and hard urethane groups, as revealed by the analysis of hydrogen bonding in Fourier transform infrared. Consecutively, the glass transition shifted to higher temperatures for these polymers. On the other hand, strong hard–hard inter‐urethane associations were observed in 2,6‐TDI‐based prepolymers; these led to microphase segregation between polyether chains and urethane groups, as revealed by optical microscopy. This inhomogeneous structure was thought to be responsible for the unusual rheological behavior of these PUA prepolymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2750–2768, 2000 相似文献
949.
采用固相亚磷酸三酯法和化学与酶促相结合的方法合成了9个PstI接头片段,并对它们进行了酶切反应研究.结果表明:PstI识别序列中尿苷的存在只降低其互补链的切割程度,对其本链没有影响;PstI切割底物是按两步单股切割机制进行,作用的最小底物是含有识别序列的8~12聚脱氧核糖核苷酸,且对底物中识别序列两边的碱基对数目有一定要求;PstI识别序列中的二个胞苷在酶反应中起的作用是不一样的 相似文献
950.
作为第三代半导体材料的典型代表,碳化硅因具备宽的带隙、高的热导率、高的击穿电场以及大的电子迁移速率等性能优势,被认为是制作高温、高频、高功率以及高压器件的理想材料之一,可有效突破传统硅基功率半导体器件的物理极限,并被誉为带动“新能源革命”的绿色能源器件。作为制造功率器件的核心材料,碳化硅单晶衬底的生长是关键,尤其是单一4H-SiC晶型制备。各晶型体结构之间有着良好的结晶学相容性和接近的形成自由能,导致所生长的碳化硅晶体容易形成多型夹杂缺陷并严重影响器件性能。为此,本文首先概述了物理气相传输(PVT)法制备碳化硅晶体的基本原理、生长过程以及存在的问题,然后针对多型夹杂缺陷的产生给出了可能的诱导因素并对相关机理进行解释,进一步介绍了常见的碳化硅晶型结构鉴别方式,最后对碳化硅晶体研究作出展望。 相似文献